# std::set<Key,Compare,Allocator>::set

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< cpp‎ | container‎ | set

| (1) |
set(); |  | (until C++11)
set() : set(Compare()) {} |  | (since C++11)
explicit set( const Compare& comp, const Allocator& alloc = Allocator() ); | (2) |
explicit set( const Allocator& alloc ); | (3) | (since C++11)
template< class InputIt > set( InputIt first, InputIt last, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); | (4) |
template< class InputIt > set( InputIt first, InputIt last, const Allocator& alloc ) : set(first, last, Compare(), alloc) {} | (5) | (since C++14)
set( const set& other ); | (6) |
set( const set& other, const Allocator& alloc ); | (7) | (since C++11)
set( set&& other ); | (8) | (since C++11)
set( set&& other, const Allocator& alloc ); | (9) | (since C++11)
set( std::initializer_list<value_type> init, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); | (10) | (since C++11)
set( std::initializer_list<value_type> init, const Allocator& alloc ) : set(init, Compare(), alloc) {} | (11) | (since C++14)
template< container-compatible-range<value_type> R > set( std::from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); | (12) | (since C++23)
template< container-compatible-range<value_type> R > set( std::from_range_t, R&& rg, const Allocator& alloc ) : set(std::from_range, std::forward<R>(rg), Compare(), alloc) {} | (13) | (since C++23)

Constructs new container from a variety of data sources and optionally using user supplied allocator alloc or comparison function object comp.

1-3) Constructs an empty container.
4,5) Constructs the container with the contents of the range `[`first`,` last`)`. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).
If `[`first`,` last`)` is not a valid range, the behavior is undefined.
6,7) Copy constructor. Constructs the container with the copy of the contents of other.

If alloc is not provided, allocator is obtained by calling std::allocator_traits<allocator_type>:: select_on_container_copy_construction(other.get_allocator()). | (since C++11)
During class template argument deduction, only the first argument contributes to the deduction of the container's `Allocator` template parameter. | (since C++23)

8,9) Move constructor. Constructs the container with the contents of other using move semantics. If alloc is not provided, allocator is obtained by move-construction from the allocator belonging to other.

During class template argument deduction, only the first argument contributes to the deduction of the container's `Allocator` template parameter. | (since C++23)

10,11) Initializer-list constructor. Constructs the container with the contents of the initializer list init.If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).
12,13) Constructs the container with the contents of rg. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).

### Parameters

alloc | - | allocator to use for all memory allocations of this container
comp | - | comparison function object to use for all comparisons of keys
first, last | - | the range to copy the elements from
other | - | another container to be used as source to initialize the elements of the container with
init | - | initializer list to initialize the elements of the container with
rg | - | a container compatible range, that is, an `input_range` whose elements are convertible to `value_type`
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `Compare` must meet the requirements of Compare.
- `Allocator` must meet the requirements of Allocator.

### Complexity

1-3) Constant.
4,5) \(\scriptsize N \cdot log(N)\)N·log(N) where \(\scriptsize N\)N is std::distance(first, last) in general, linear in \(\scriptsize N\)N if `[`first`,` last`)` is already sorted by value_comp().
6,7) Linear in size of other.
8,9) Constant. If alloc is given and alloc != other.get_allocator(), then linear.
10,11) \(\scriptsize N \cdot log(N)\)N·log(N) where \(\scriptsize N\)N is init.size() in general, linear in \(\scriptsize N\)N if init is already sorted by value_comp().
12,13) \(\scriptsize N \cdot log(N)\)N·log(N) where \(\scriptsize N\)N is ranges::distance(rg) in general, linear in \(\scriptsize N\)N if rg is already sorted by value_comp().

### Exceptions

Calls to `Allocator::allocate` may throw.

### Notes

After container move construction (overload (8,9)), references, pointers, and iterators (other than the end iterator) to `other` remain valid, but refer to elements that are now in *this. The current standard makes this guarantee via the blanket statement in [container.reqmts]/67, and a more direct guarantee is under consideration via LWG issue 2321.

Although not formally required until C++23, some implementations has already put the template parameter `Allocator` into non-deduced contexts in earlier modes.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | `202202L` | (C++23) | Ranges-aware construction and insertion; overloads (12,13)

### Example

Run this code

    #include <cmath>
    #include <iostream>
    #include <set>
    #include <string>

    struct Point { double x, y; };

    struct PointCmp
    {
        bool operator()(const Point& lhs, const Point& rhs) const
        {
            return std::hypot(lhs.x, lhs.y) < std::hypot(rhs.x, rhs.y);
        }
    };

    std::ostream& operator<<(std::ostream& os, Point pt)
    {
        return os << '(' << pt.x << ',' << pt.x << ')';
    }

    void println(auto rem, const auto& seq)
    {
        std::cout << rem << '{';
        for (auto n{seq.size()}; const auto& elm : seq)
            std::cout << elm << (--n ? ", " : "");
        std::cout << "}\n";
    }

    int main()
    {
        // (1) Default constructor
        std::set<std::string> a;
        a.insert("horse");
        a.insert("cat");
        a.insert("dog");
        println("1) a: ", a);

        // (4) Range constructor
        std::set<std::string> b(a.find("dog"), a.end());
        println("2) b: ", b);

        // (6) Copy constructor
        std::set<std::string> c(a);
        c.insert("another horse");
        println("3) c: ", c);

        // (8) Move constructor
        std::set<std::string> d(std::move(a));
        println("4) d: ", d);
        println("5) a: ", a);

        // (10) Initializer list constructor
        std::set<std::string> e{"one", "two", "three", "five", "eight"};
        println("6) e: ", e);

        // Custom comparison
        std::set<Point, PointCmp> f = {{2, 5}, {3, 4}, {1, 1}};
        f.insert({1, -1}); // This fails because the magnitude of (1,-1) equals (1,1)
        println("7) f: ", f);

        // (12) Range constructor
        const auto w = {"Eurybia", "Theia", "Rhea", "Aura", "Mnemosyne", "Mnemosyne"};
    #if __cpp_lib_containers_ranges
        std::set<std::string> g(std::from_range, w); // overload (12)
    #else
        std::set<std::string> g(w.begin(), w.end()); // fallback to (4)
    #endif
        println("8) g: ", g);
    }

Possible output:

    1) a: {cat, dog, horse}
    2) b: {dog, horse}
    3) c: {another horse, cat, dog, horse}
    4) d: {cat, dog, horse}
    5) a: {}
    6) e: {eight, five, one, three, two}
    7) f: {(1,1), (3,3), (2,2)}
    8) g: {Aura, Eurybia, Mnemosyne, Rhea, Theia}

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2076 | C++11 | overload (4) conditionally required `Key` to be CopyInsertable into *this | not required
LWG 2193 | C++11 | the default constructor was explicit | made non-explicit

### See also

operator= | assigns values to the container (public member function)

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